The present invention is directed generally to the measurement of certain optical properties of the interaction of light with particulate matter in a fluid.
Power generation plants and other industrial facilities may release particulate matter into the atmosphere, especially if the facilities burn coal or other fossil fuels. Particulate matter in the flue gases generated from the burning of coal tends to be mostly spherical, solidified droplets of a variety of sizes from the metallic components of the ash in the coal. Because the particulate matter degrades air quality and reduces visibility (haze) governmental regulations require some facilities to continuously control and monitor their particulate matter emissions. Particulate mass emission rates are reported in units of weight per time (tons/yr), and are determined from particulate mass loading measurements in wt/volume (grains/standard cubic foot) and a corresponding measure of stack gas flow rate in volume/time (standard cubic feet/hour). For example, Environmental Protection Agency (EPA) or state regulations require some sources to continuously monitor the mass loading of their particulate matter emissions to prove compliance with particulate emission limitations.
Although a number of techniques are available to measure the mass loading of particulate matter emissions directly, many are costly and incapable of reliable continuous operation under field conditions. Therefore, continuous indirect measurements of optical or other properties related to the mass loading are often used in association with a correlation coefficient or a transfer function that allows calculation and output of an equivalent particulate mass loading value. Recently, the EPA implemented Performance Specification 11 (40 C.F.R. § 60, Appendix B, PS-11) to define how a particulate monitor is to be tested, and the resulting performance that is required to “certify” a particulate monitor. Certification is a term designating that a specific monitor meets the relevant EPA performance requirements for that type of analyzer and can be used to report emissions for determination of the compliance status of a given emission source. PS-11 requires that properties of particulate matter measured by continuous techniques be correlated with, or converted to, particulate mass loading and compared to a reference method (40 C.F.R. § 60, Appendix A, RM 5 or 17, for example) measurement, or series of measurements. Reference method measurements for particulate mass loading use a manual stack sampling technique and are non-continuous measurements. Several different characteristics of the interaction of light with particulate matter can be measured and correlated with particulate density, including those discussed below.
Extinction is a measure of the quantity of optical energy absorbed or scattered by particulate matter. Extinction may be measured over many wavelengths of light, however, some extinction measurements, called opacity measurements, require light spanning the visible range (400 to 700 nm). Extinction measurements are dependent on variables other than mass loading, thus reducing the accuracy of an extinction measurement as a predictor of particulate mass loading. For example, extinction is dependent on particulate shape and size, with the strongest response (for a given mass loading) occurring when particulate diameter is comparable to the wavelength of the light source. Given constant particle size distribution, shape and specific gravity, the extinction response is closely correlated with mass loading. Extinction measurements may be unusable at very low particulate mass loadings as the extinction measurement becomes difficult to make with good accuracy. The specific gravity of the particulate matter itself also affects the correlation of the extinction measurement with particulate mass loading because the optical properties of the particulate is caused by the surface area of the particulates and not the material inside the particles.
FIG. 1 depicts a prior art device 100 for measuring extinction caused by particulate matter 128 in a fluid 116, which may be an exhaust stream in an exhaust stack 120. In the device 100, a light source 102 directs a forward light beam 112 through a beam splitter 106 and lens 108 toward a reflector 110. The reflector 110 receives the forward beam 112 and reflects a return beam 114 toward a detector 104 after reflecting the return beam 114 off of a beam splitter 106. The beams 112, 114 propagate across the fluid 116 through holes 130, 132 in the stack 120. A large portion of the optical energy present in the beams 112, 114 reaches the detector 104; however, some optical energy is absorbed or scattered by the particulate matter 128 present in the fluid 116. The difference between the optical energy emitted by the light source 102 and detected by the detector 104 with a clear path (no particulate in this case) and the optical energy received by the detector 104 with particulate present in the optical path indicates the extinction caused by the particulate, thereby providing an indication of the amount of the particulate 128 present in the fluid 116. Since the beam goes across the stack and back, these optical systems are also known as double-pass.
Scatter is a measure of the quantity of optical energy that is scattered by particulate matter, or reflected off the axis of the interrogating light beam. Devices for measuring scatter may direct a light beam across an exhaust stream and measure the quantity of light that is scattered at different angles away from the beam's expected propagation direction. Backscatter refers to scatter near 180 degrees away from the projected beam, and forward scatter refers to scatter near the same angle as the projected beam. Different types of scatter measurements detected at a variety of angles, light source wavelengths and beam widths display different degrees of dependence on particulate properties other than mass loading.
Backscatter, for example, measures optical energy reflected by particulate matter in a backward direction compared to the direction of the projected beam. Backscatter is somewhat less dependent on particulate size than extinction, but still responds most strongly to very small particles. Also, unlike extinction, backscatter provides a strong response when particulate mass loading is low. Like extinction, however, backscatter techniques are very sensitive to the size, and shape of the particulate. Backscatter is also sensitive to particulate color.
FIG. 2 depicts a prior art device 200 for measuring backscatter in the fluid 216. In the device 200, the light source 202 directs a forward beam 212 through a beam splitter 206 and lens 208 across the exhaust stack 220 through holes 230, 232. An absorbing device 218, i.e. an optically black material, positioned opposite the light source 202 prevents the forward beam 212 from being reflected back to the detector. Even though the forward beam 212 is not reflected by the absorbing device 218, optical energy from the forward beam 212 is still scattered back to the detector 204 by particulate 228 present in the fluid 216 after reflecting off of beam splitter 206. The amount of optical energy received by the detector 204 is indicative of the backscatter caused by the particulate, thereby providing an indicator of the quantity of the particulate 228 in the fluid 216.
Near angle forward scatter is a measurement of the optical energy that is scattered by particulate matter in small, near forward angles. Like backscatter, near forward scatter provides a strong response when particulate mass loading is low. Unlike extinction and backscatter, however, near forward scatter provides a strong response for relatively large particulates with diameters greater than 2 microns with a visible light source. In addition, near forward scatter techniques minimize the undesirable color, size, and shape dependencies of extinction and backscatter measurements.
Existing devices for measuring near forward scatter are often complicated, expensive, and difficult to calibrate and maintain. Some require beam steering as well as active optical equipment on both sides of the exhaust stream. Many are point sampling or close coupled extractive devices and hence introduce measurement errors in applications with significant particulate stratification. Existing devices are also unable to take advantage of the strengths of other known techniques, for example extinction and backscatter. Consequently, there exists a need for a simple, efficient way to measure near angle forward scatter. In addition, there exists a need for a technique to implement multiple measurement methods with a single device so that the correlation of a group of optical measurements with particulate density can be made more independent of the size, shape, and color of the particulate matter itself than can be obtained with a single measurement.